Channel Quality Indicator (CQI) is frequently used for precoding, link adaptation and other radio resource management algorithms in wireless communication systems. In wideband systems, such as e.g. Long-Term Evolution (LTE), finer frequency granularity for CQI can lead to better channel dependent scheduling and link adaptation, thus resulting in higher throughput. However, fine frequency granularity will cause a big feedback overhead for CQI report, and CQI compression methods are employed to save signalling overhead, such as best-M or user equipment (UE)-selected besides the wideband CQI reporting. In LTE, the downlink CQI can be reported in two kinds of feedback channels: Physical Uplink Control Channel (PUCCH) and Physical Uplink Shared Channel (PUSCH). PUCCH CQI is a periodic resource allocated from the eNB, or base station as it also may be referred to, and it does not need scheduling trigger, while PUSCH CQI is aperiodic, and it relies on the signalling from the base station to indicate when, where and how to report. Different patterns, in terms of CQI mode, define that the wideband or partial band CQI, are reported via PUCCH with different periodicities or PUSCH based on scheduling grants. It may be noted that it is completely up to the eNB to configure CQI reporting resources on PUCCH and to determine how and when to ask for CQI reports on PUSCH. None of these reports are mandatory.
In LTE release 8, is Multimedia Broadcast Single Frequency Network (MBSFN) specified. MBSFN is a transmission mode which exploits the Orthogonal Frequency Division Multiplexed (OFDM) radio interface to send multicast or broadcast data as a multicell transmission over a synchronized single frequency network. The transmissions from the multiple cells are sufficiently tightly synchronized for each to arrive at the user equipment within the OFDM Cyclic Prefix (CP) so as to avoid Inter-Symbol Interference (ISI). In effect, this makes the MBSFN transmission appear to a user equipment as a transmission from a single large cell, dramatically increasing the Signal-to-Interference Ratio (SIR) due to the absence of inter-cell interference.
Moreover, in a system which supports type I relay, the MBSFN configuration is adopted in the access link in the relay cell for backhaul downlink in the anchor cell. In such a way, relay nodes will configure the MBSFN subframe in the relay cells so that user equipment that has detected the MBSFN configuration will not receive any data in the rest PDSCH. On the other hand, in the backhaul link, downlink data will be delivered from anchor-eNB to relay nodes during such a MBSFN subframe. The latest agreement in 3rd Generation Partnership Project (3GPP) shows that the configuration of backhaul and access links in time domain is semi-persistent, i.e. the MBSFN configuration in relay cells is rather fixed in a long time scale and known to eNB and relay nodes respectively.
In a downlink system supporting self-backhauling or type I relay, interference coming from other relay cells, which can be regarded as inter-cell-interference, dominates in the anchor cell. Therefore, the interference variation from the relay cells is very important to the anchor cell. Configuration of MBSFN in the relay cells can bring significant interference variation to the relay nodes and macro-user equipments of the anchor cell: during the MBSFN subframe of relay cells, the relay nodes will not perform any data transmission but only the control signalling indicating the MBSFN configuration, i.e. the main interferers i.e. the relay nodes, will mute, whereas during the normal downlink subframe, eNB and relay nodes might transmit at the same time. So this would lead to some semi-static interference variation when it is agreed that the MBSFN configuration on relay cells is rather semi-static or fixed in a long time scale.
CQI in the anchor cell will be used either for the backhaul or normal downlink transmission, while CQI measurement of the anchor cell took place some time ago, in terms of CQI delay including propagation and processing delay. Thus the CQI in the anchor cell may be altered between backhaul and normal downlink transmission either during the normal subframe transmission, corresponding to the normal downlink subframe in relay cells as well, or during the backhaul subframe transmission, corresponding to the MBSFN subframe in relay cells.
In the first case, there are simultaneous data transmission in the anchor cell and the relay cell, and the interference from the neighbouring relay nodes dominates. While in the second case, the data transmission in the relay cell is muted by MBSFN configuration, so the interference from such neighbouring relay node is null. This can be illustrated in FIG. 1a and FIG. 1b. 
FIG. 1a illustrates downlink transmission in normal mode, i.e. normal subframe transmission wherein the 110 and 120 transmit signals simultaneously which may cause signal interference at the user equipments 130.
FIG. 1b illustrates downlink transmission in backhaul mode, i.e. backhaul subframe transmission, corresponding to the MBSFN subframe transmission in relay cells.
As the timing of transmission based on these CQI also fall into the two cases, either the normal downlink subframe mode or the backhaul downlink subframe mode. This means that it is likely to have subframe mismatch of the measurement and the transmission. This would lead to CQI accuracy degradation and resulting performance degradation.